The best season to visit Manali is during the long dry season, which runs from April to July. These are considered the best months for Local sight seen, Rohtang Trip, trekking and Adventure vacations in Manali.
Of course, these months are the high season of travel. They will be busier and some accommodation, hotels and taxi will apply higher rates.
If you prefer a more isolated experience, consider visiting Manali during the short rainy season from late October to December.
If possible, avoid traveling to Manali during the long rainy season, from August to September. The rains are usually intense and sudden, which can cause the cancellation of activities at the last minute.